Coil spike suppression diode

Technical discussion on converting internal combustion to electric
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soyachips
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Coil spike suppression diode

Post by soyachips »

Hi there,

What are the differences between using a single diode for coil spike suppression vs. two zener diodes as suggested on the EV Works site?

http://www.evworks.com.au/tech/contacto ... /index.php

Also for the pre-charge resistors, they've used two smaller ones instead of one bigger one. Would this be for cost or size or is there another reason?

Thanks!
Andrew
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Johny
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Coil spike suppression diode

Post by Johny »

soyachips wrote:What are the differences between using a single diode for coil spike suppression vs. two zener diodes as suggested on the EV Works site?
I notice that many contactor manufacturers indicate that the opening time of the contactor is longer (worse) when diode back-emf suppression is used. Longer opening time of contactors (if under load) would lead to more contact burning. The Zener approach would be better in this regard. If the contacts are not carrying load when they open, I'd use a single diode.
soyachips wrote:Also for the pre-charge resistors, they've used two smaller ones instead of one bigger one. Would this be for cost or size or is there another reason?
Cost I would think. There may be a slight advantage in dissapation with 2 resistors rated half of one big one but it would be marginal.
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Coil spike suppression diode

Post by bga »

Zener... I'd go for the speed of opening argument.

The suppressor is designed to reduce a potentially damaging high voltage spike that occurs when the coil is de-energised. The coil is an inductor and stores energy in the form of the magnetic field. In order to open the contactor, the magnetic field has to be removed from the coil. As the field is reducing, the voltage across the contactor will be proportional to the rate of reduction of the field. Simply opening the supply to the coil will cause a huge voltage spike as there is nowhere for the energy in the coil to go.
[this energy is often dissipated in the transistor as a avalanche breakdown leakage when the maximum voltage (VceO max) of the transistor is exceeded. It can also kill the transistor]

A reverse diode conducts when the contactor is de-energised, causing the magnetic field to decay more slowly as the energy is dissipated in the diode and the windings of the inductor.

A zener diode and a diode/resistor combination do essentially the same thing except that they dissipate the energy more quickly.

Adding a resistor in series with the diode increases the voltage across the suppressor and allows energy to be also dissipated in the resistor, speeding up the field reduction process and gence opening of of the contactor. This inproved speed comes as the expense of a higher transient voltage on the coil terminal, which should not be an issue, provided it doesn't cause problems for any of the components.

A typical resistor value for a small coil is about 100 ohms. A 500mA coil current will cause about a 50 volt (V=IR) spike in this reistor, which is far better than the 500 or 1000 volt spike from an open circuit. The design issue is that the transistors can withstand the trainsient voltage, headroom of 2x is good.

It's important to make sure that the diode, zener or resistors are able to absorb the energy from the inductor. Peak current is important for occasional operation, power may be an issue for fast repetitve operation.

A reason that you may choose a diode/resistor over a zener would be that of speed, ultra-fast diodes can be used so that minmal overshoot is experienced before the diode starts to conduct. (a small capacitor can do help to reduce this)

Probably too much info...

useful page here

[edit: add link]
Last edited by bga on Wed, 02 Sep 2009, 13:49, edited 1 time in total.
soyachips
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Coil spike suppression diode

Post by soyachips »

Thanks for the great replies. I also found the following information useful:

http://en.wikipedia.org/wiki/Transorb

I read somewhere that the size of the diodes should be based on the size of the contactor (bigger coils can store more energy), so for a contactor rated at 400A, will 2 x 1N5352B (15V 5W) Zener diodes be sufficient?
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Post by Johny »

It really depends on the coil current for the contactor, not the contact rating. If I were using a diode/resistor (my preference over zeners) I would use one with the same current rating the the coil current. For zeners it will be less but frankly I'm surprized that Zeva's zeners don't expire on the first opening.
Any comments bga?
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Coil spike suppression diode

Post by bga »

Hi Johnny,

I would agree that Zeners for this are not being used according to the maker's reccomendations. A good analog for a typical Zener diode is a tranzorb in series with a 25 ohm resistor.
The tranzorb has a very flat V-I relation and behaves like an 'ideal' zener diode.

Most coils have a lot of resisitance in the copper of coil so the holding current isn't very high. Initially, transient spike current will be equal to this and then decay away, so the zener only has to ba able to handle this current for a short time. I suspect that this is why zeners don't blow up.

Cheers
BGA
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Coil spike suppression diode

Post by soyachips »

Thanks for the info guys. It sounds like diode/resistor is the way to go but I don't really understand how to work out which diode (ultrafast?) and resistor I need. I've had a look on the contactor manufacturer's site but I'm not sure what to look for.

http://www.kingnan.com/en/product_show.asp?id=75

Any chance I could get your help?

Let me know if you need any other info.

Thanks!
Andrew
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coulomb
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Post by coulomb »

soyachips wrote: It sounds like diode/resistor is the way to go but I don't really understand how to work out which diode (ultrafast?) and resistor I need.

My understanding (and I'm more than willing to be corrected here) is that the diode doesn't need to be terribly fast. So a 10c 1N4004 (1 A 400 V) would probably do it.

A resistor about the same resistance as the coil would possibly be a starting point. So a coil that draws say 500 mA from 12 V (really 14 V) would be about 24 ohms, so try a 22 ohm (preferred value) at say 1W. The diode will have almost 1 volt drop across it when conducting hard. The resistor will dissipate some 7W briefly, so I wouldn't use smaller than 1W, but it's for such a short time (milliseconds) that 1W will probably do.

Perhaps a 3 A diode would be better; 1 A doesn't leave much headroom, and many contactor coils draw more than half an amp.

Edit: clarify what type of coil.
Last edited by coulomb on Sat, 05 Sep 2009, 06:01, edited 1 time in total.
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Coil spike suppression diode

Post by bga »

Agreed.

The diode speed isn't critical. Probably the smaller diode will be fine, a 1N4004 can carry a 30 amp peak (8.3ms 1/2 sine wave), it's limited by power dissipation. [A 1N5404 (3Amp) is rated for a 200 amp peak]

1N4004s don't usually specify any speeds and are slow compared to the high speed types. I have seen 1N4004s used as diverters in lightning arrestors, so they're not really slow.

I think that the resistor value being equal to the coil resistance is a good rule of thumb for low voltage coils. For high voltage coils, the overshoot should be considered and probably a smaller resistor value is needed to prevent excessive overshoot killing transistors.

Diode speed is important for switching circuits where the diode conducts and blocks thousands of times per second. Doides have a certain amount of 'inertia' and it takes then a little time to either start or stop conducting. Contact closure events are only infrequent so this can be largely ignored.

Cheers
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Coil spike suppression diode

Post by soyachips »

Fantastic ... thanks to all for the advice!
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300zxev
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Coil spike suppression diode

Post by 300zxev »

Do we need a diode for tyco ev200 contactors ?
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Coil spike suppression diode

Post by Tritium_James »

No, they have a built-in suppression circuit as part of their power minimiser module.
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Coil spike suppression diode

Post by bga »

There might be a gotcha on EV200 contactors:

There's also an LEV200 which doesn't have the economiser and is a larger package. Both look to have the same contact properties.

LEV200 $89 (12watts hold power) at evsource.com
EV200 $139 (1watt hold power) at evsource.com



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